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Icom VHF-Receiving but not transmitting far away

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Icom VHF-Receiving but not transmitting far away

Old 11-14-2018, 03:40 AM
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Default Icom VHF-Receiving but not transmitting far away

Was hoping someone could point me in the right direction. I have an Icom 502 VHF radio attached to an 8' Shakespeare antenna mounted on the hard top of a 34' SeaVee. Recently, I become concerned about the distance I could transmit on a recent trip to the Bahamas. So I had a friend call me on his VHF from about 8-9 miles away and I could hear him crystal clear, but he could not hear me at all...verified using text messages. To check a more intermediate distance, I called a bridge operator that is ~2-3 miles away and he could hear me no problem. So, I believe the radio can transmit and receive with no issue for short distances but concerned about longer distances. Channel 24 (or 27, can't remember) used to allow you to call and it would return a message but that system doesn't appear to be working anymore so I can't use that as a reliable source to pinpoint the mileage of drop off.

My guess is the antenna should be ok given I can receive and should I assume the VHF has an issue? Any other ways to test the antenna vs. the VHF to isolate the issue? (Yes, transmitting at 25W) Thanks for any advice...​​​​​​
Old 11-14-2018, 04:13 AM
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Poor Standing Wave Ration (SWR) will effect transmission output. The length of the coax, proximity of antenna to other conductive materials, and other things can effect the SWR of the antenna. You can buy SWR and power meters that plug in between the radio and coax. Most of these are used to tune the antenna and then removed once everything is set up. The SWR should be as low as possible. You probably won't get below 1.5 so that's a good target. A SWR of 2 is fine. Getting close to 3 means some adjustment is needed.



CB antennas often have a screw that can be adjusted at the tip to adjust the SWR. I've never seen this on a marine radio so I'm not sure what you would do with a high SWR other than replace the antenna.
Old 11-14-2018, 04:22 AM
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Antenna's with more "gain" will also transmit farther. An antenna with a 3dB gain is pretty standard and transmits in a sphere. Equal transmission power goes in all directions - including toward the sky and toward the ground - where the signal isn't very useful.

As the antenna gain increases, the radiated field looks more like a disk with the energy transmitting horizontally in all directions with less energy being transmitted up and down. The same 25W output goes farther because it's being focused toward the horizon. a 9dB antenna transmits in a fairly flat disk which means more power focused horizontally. But, the antenna and boat need to be vertical for this to work. If you like the swept back antenna look, a 9B antenna will transmit to the sky in front of you and into the water behind you. A rocking boat will point the signal at the sky, horizon, water, horizon, sky, etc.

A middle of the road antenna will be a 6dB. If your antenna is 8' long, it's likely a higher gain antenna but not necessarily.
Old 11-14-2018, 05:28 AM
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Antenna problems can be masked on reception because the receiver typically has tremendous reserve gain it can use to overcome the poor antenna performance. On transmit the gain is fixed, so a problem in the antenna causes the radiated signal to be much lower than normal.

The most direct way to assess the transmitter is to measure its power output. You will need an in-line wattmeter for that. Typically these in-line wattmeters are also directional wattmeters, so you can also assess the standing wave ratio on the transmission line with the same test instrument.

You could make a more informed inference about the antenna while receiving by tuning to a NOAA Weather Broadcast from a distant station. Typically in any area of the USA there are several NOAA stations within range of a properly working receiver and antenna. If you can hear a NOAA station that is 70-miles away, then the antenna is probably working properly. If you cannot hear any long-distance NOAA stations, the receive test is inconclusive.

As for antennas, the most important factor is the height above the sea. A higher antenna of less gain is preferred over a lower antenna with higher gain. Height beats gain.
Old 11-14-2018, 05:37 AM
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I have seen and corrected this problem several times.
I assume that your VHF radio was transmitting OK at some time in the past and is now showing the symptoms of OK reception and poor transmission....

The fix (every time I've seen this) is to check your coax connections. You may get by with just cleaning them up and wiping them with some dielectric grease. If that doesn't fix the problem replace the coax connectors, it's not difficult or expensive.
Old 11-14-2018, 05:43 AM
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Originally Posted by TDuffy35 View Post
...check your coax connections...wiping them with some dielectric grease.
Dielectric grease is a super insulator. It does not restore electrical conductivity, it prevents electrical conductivity.

Last edited by jhebert; 11-14-2018 at 06:30 AM.
Old 11-14-2018, 05:51 AM
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Thanks for the input here, appreciate the help. I did inspect the connection points and they look to be in good condition but might replace just to be sure. I think my next best step is to get a VHF tester to test wattage and SWR. Working with some friends to see if they have one, if not, think it is a good tool to have. The one thing I have not heard to this point is, it might be the Tx in the radio? My experience with Icom radio is if well cared for, they are bullet proof...anyone have a different experience with Icom?
Old 11-14-2018, 06:20 AM
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Icom radio issue, send it in for repair. My 504 suffered fr same problem., unfortunately this problem is not that uncommon.
i tried troubleshooting everything then finally figured must be the radio. Fortunately i have 2 VHF's with an 8 foot whip for each and so other than a frustrating weekend trying to figure why I wasn't transmitting on the one radio, i still like icom, albeit next radio likely Std horizon model with AIS.
Old 11-14-2018, 06:32 AM
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Originally Posted by jhebert View Post
Dielectric grease is a super insulator. It does not restore electrical conductivity, it prevents electrical conductivity.
You're incorrect. DG is a great inhibitor of salt water corrosion and is recommended for marine electrical applications.
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Old 11-14-2018, 06:34 AM
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Originally Posted by moebur1 View Post
...[could the cause of the problem in transmitting] be the Tx in the radio?
Yes. That is possible. You should consider that possible cause. A modern solid-state transmitter output section would be expected to have a very long service life, but if the radio were allowed to transmit into an improper load for long periods, the output transistors could become too hot. Too much heat leads to failure.

A poor antenna, a bad connection, a problem in the transmission line could all affect the load presented to the transmitter. In the event that you find the transmitter to have a failed output stage, you should also carefully check the transmission line, all connections, and the antenna before replacing the transmitter.

Also, transmitting requires much higher DC power than receiving. If there is a problem in the power distribution wiring to the radio, the ability of the radio to transmit can be affected. Poor DC wiring to the radio could also be a cause of reduced transmitter output power. You can check this with an accurate DC voltmeter connected to the radio power cord to read the actual DC power at the radio when transmitting.

You can also make an inference about the radio transmitter power output by measuring the DC current being drawn during transmit. This can be done more easily than to measure the radio-frequency power output of the radio. Most modern digital multi-meters can measure up to 10-Amperes of DC current. Insert an Ammeter into the radio power leads. Measure the DC current during transmit at the high-power setting. The current must be around 5-Amperes or more to indicate that the radio transmitter is trying to operate at full power.

Also, modern radios typically will not transmit at full power into a bad load. The radio might be sensing that its load is improper and reducing its output power to protect itself. This could also cause the situation you report.

There are many suspects in this mystery. You will have to sort out the real cause.

One method of testing is by substitution of known-good components into the system. For example, connect the radio to a known-good load, which can be another antenna and transmission line or a dummy load. Then assess the transmitter output into the known-good load. Or, connect a known-good radio to the existing transmission line and antenna. There is some risk in that approach. If the existing antenna and transmission line are bad, the new transmitter could be damaged by them.

Last edited by jhebert; 11-14-2018 at 06:54 AM.
Old 11-14-2018, 06:57 AM
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Originally Posted by TDuffy35 View Post
You're incorrect. DG is a great inhibitor of salt water corrosion and is recommended for marine electrical applications.
Sorry. Dielectric grease is exactly as I described it. It is a super insulator. It has no chemical restorative properties as a cleaner agent. You new claims are not in conflict with mine.

However, no professional radio installers I know have ever slathered dielectric grease on the contacts of coaxial connectors. Your advice is contrary to professional installation practices. Connectors are assembled dry, and weather-proofing is applied after assembly.
Old 11-14-2018, 07:28 AM
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Thanks jhebert, will check current and voltage to see if DC could be my culprit. Its a 10 year old radio, antenna appears to be newer, but hard to say, I'm third owner.
Old 11-14-2018, 07:34 AM
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Originally Posted by jhebert View Post
Dielectric grease is a super insulator. It does not restore electrical conductivity, it prevents electrical conductivity.
True but it helps to prevent corrosion which is conductive and shorts the signal to shield.
Old 11-14-2018, 07:38 AM
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Originally Posted by jhebert View Post
Sorry. Dielectric grease is exactly as I described it. It is a super insulator. It has no chemical restorative properties as a cleaner agent. You new claims are not in conflict with mine.

However, no professional radio installers I know have ever slathered dielectric grease on the contacts of coaxial connectors. Your advice is contrary to professional installation practices. Connectors are assembled dry, and weather-proofing is applied after assembly.
I have used it for 50 plus yrs, both Motorola and Antenna Specialist provide silicone grease in their antenna installation kits. Both mobile and Base. Same with coating the + battery connections, when tightened properly, there is a metal to metal contact.
Old 11-14-2018, 07:54 AM
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There is no way to isolate the problem without a VSWR meter and 50 ohm dummy load. You may take the radio to a local radio repair shop where they can check the modulation, power, frequency and receiver sensitivity. If you can normally receive several NOAA Weather stations and can't at this time, it is a good (not perfect), indication of a poor antenna. If you can get to the antenna cable in a protected area, peel back the insulation, 2 or 3 inches max. If the shield is corroded, it means the cable got wet, and needs to be replaced. I just got done replacing 75 ft of marine RG 213 that was damaged. The water had run down the entire 55 ft of mast and 10 feet inside the cabin.
After you get done, go to H.D. and get a can of 3M sealant in the electrical section. It is $30 bucks or so, but well worth it. Be careful, this stuff is quite runny, any breeze will blow it around.
Old 11-14-2018, 11:16 AM
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Originally Posted by jwelk View Post
I have used [dielectric grease] for [more than] 50 [years] both Motorola and Antenna Specialist provide silicone grease (sic) in their antenna installation kits.
I suspect that you are talking about the assembly of the antenna itself, not a transmission line connector like a UHF connector.

Silicone grease is not sold as a dielectric grease. Silicone grease is used as a lubricant. Its dielectric characteristics are not typically specified.

When assembling antenna components, particularly ones made of aluminum, it is common to use a conductive paste or grease. I have used conductive paste or grease for 50-years myself when assembling antennas. Conductive grease is often used on electrical wiring connectors. A product called Penetrox-A is typical.

If you put dielectric grease on the contacts of electrical connectors you get greasy contacts that tend to accumulate dirt. Most electrical contacts in connectors are specified to be assembled when dry and clean. What you do after you assemble them is up to you.

Here is a good article on installing connectors:

https://www.arrl.org/files/file/Tech...df/9105034.pdf

Regarding UHF series connectors, it says:

"UHF Plugs are not waterproof. Used outdoor they allow water ingress that degrades their associated cable over time. If you use UHF connectors outside, cover them with electrical tape, Coax-Seal [a product name], or other weather proofing materials to inhibit this process."

Last edited by jhebert; 11-15-2018 at 11:18 AM.
Old 11-14-2018, 11:38 AM
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Originally Posted by jwelk View Post
...You may take the radio to a local radio repair shop where they can check the modulation, power, frequency and receiver sensitivity...
Such testing--if you can even find it locally--might cost more than a new radio. You can buy a new VHF Marine Band fixed-mount Class-D DSC radio for barely more than $100 by discounted pricing and rebates.
Old 11-14-2018, 02:22 PM
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Might sound silly but Icom mikes are sensitive to how you talk when transmitting. Make sure you're holding the mike 1-2" from your mouth, talk in a normal tone, and key the mike a full second before talking. I had an Icom 402 that didn't transmit well until I made that adjustment.
Old 11-14-2018, 02:48 PM
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Originally Posted by moebur1 View Post
[It is] a 10 year old radio...I'm third owner.
There is a good chance that the radio is an older DSC radio that is qualified only to the (obsolete) RTCM SC-101 specifications. A newer radio, qualified to the DSC Class-D specification that is now required, will be a significant improvement in the digital selective calling behavior of the radio. A new, fixed-mount, 25-Watt, DSC Class D radio will only be in the range of $120. Considering that your VHF Marine Band radio is most likely the most important safety device on the boat, you should not hesitate to upgrade the radio to a modern Class-D radio.
Old 11-14-2018, 05:09 PM
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Originally Posted by moebur1 View Post
...and I could hear him crystal clear, but he could not hear me at all...verified using text messages. ...​​​​​​
Maybe your friend has a receiver problem.

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